The present invention relates to the fabrication of dielectric members using mica, and more particularly to the fabrication of laminations of mica and conductive materials.
Mica has long been known by those skilled in the art to be a suitable dielectric material for use in many different applications, including the construction of capacitors. Mica possesses superior dielectric properties, including a high dielectric constant and good dielectric strength. As a stable, inorganic material mica also has the advantage that it will resist eroding by a number of different substances. Mica may be easily fabricated in thin, uniform dielectric layers with thicknesses in the range of 0.25 to 1.0 mil. When fabricated in this thickness, mica is an extremely sturdy, durable material.
A particularly common application for mica is the use of mica as a dielectric component of capacitors. Mica capacitors are normally constructed by "silvering"--that is by printing electrodes onto blades of mica, usually by means of a silk screen process. The silver is applied to the mica in a solution, and the solvent evaporated by firing the combination in an oven. This fabrication method provides a good connection between mica and electrode, and allows a compact design by avoiding thick blades or foils. Because of the delicate nature of the electrodes created with this process, it is necessary to completely encapsulate the mica-electrode laminate to protect the electrodes from environmental influences.
In certain utilizations, however, it is necessary to directly expose the mica dielectric and electrode material to air. One such utilization is shown in U.S. Pat. No. 4,155,093, which discloses apparatus for generating ions in air. With reference to the prior art view of FIG. 1, the ion generator 10 comprises two conducting electrodes 12 and 13 separated by a layer 11 of mica. When a high frequency electrical field is supplied between these electrodes by source 14, a pool of negative and positive ions is generated in the areas of proximity of the apertured electrode 13 and the surface of the mica. Thus, in FIG. 1, an air gap breakdown occurs relative to a region 11-r of dielectric 11, creating an ion pool in hole 13-h which is formed in electrode 13.
These ions may be used, for example, to create an electrostatic latent image on a dielectric member 15 with a conducting layer 16. When a switch 18 is switched to position X and grounded as shown, the electrode 16 is also at ground potential and little or no electric field is present in the region between the ion generator 10 and the dielectric member 15. However, when switch 18 is switched to position Y, at which the potential of the source 17 is applied to the electrode 13, this provides an electric field between the ion reservoir 11-r and the back of dielectric member 15. Ions of a given polarity (in the generator of FIG. 1, negative ions) are extracted from the air gap breakdown region and charge the surface of the dielectric member 15. The rate of charging the dielectric surface may be expressed as a given ion current.
The ion generator shown in FIG. 1 requires exposure of the mica 11 and the apertured electrode 13 to air. As such, it has been found that fabricating mica laminates by silvering results in laminations which are unable to withstand the incursion of materials, such as ozone and nitric acids, which are produced as normal byproducts of the ion generation process. On the other hand, traditional methods of laminating thicker layers of conducting foils, such as bonding the layers with thermoset adhesives, present the problem that mica is easily delaminated, particularly when subjected to the action of liquids.
Although known encapsulation techniques for mica capacitors protect against delamination due to moisture, these techniques are unsuitable for applications which require exposure to air of the conductive material as well as the dielectric. The construction of an externally exposed mica-foil laminate by traditional methods will result in a structure which will tend to deteriorate easily and have only a very short service life. A laminate of the type illustrated in FIG. 1 must withstand high peak voltage RF signals, on the order of kilovolts.
Accordingly, it is a principal object of the invention to provide a method of fabricating durable mica-conductor laminates. A related object is that the laminates of the invention resist delamination due to moisture, and erosion due to ozone, nitric acid, and other substances.
Another object of the invention is the achievement of a mica-conductor laminate which exposes the various layers to air. A related object is the design of a laminate which is suitable for generating ions in air.
Yet another object of the invention is the fabrication of a mica-conductor laminate which is physically stable over a wide range of temperatures. A related object is the achievement of an ion generator which can carry high peak voltage RF signals over a long service life.